The present invention relates to a method and apparatus for laterally positioning either the lower and/or upper end of a fuel assembly within a core of a nuclear reactor.
Fuel assemblies forming the nuclear core of nuclear reactor are generally comprised of a plurality of elongated fuel rods containing fuel or fissionable material which are grouped and joined together into rectangular fuel assemblies. Each of these rectangular fuel assemblies must be precisely positioned and aligned within the core within predetermined acceptable tolerances. Accordingly, means must be provided within the nuclear reactor vessel to support the nuclear core and the individual fuel assemblies making up the core and to align and position each of these fuel assemblies.
Recently, nuclear reactor fuel assemblies have been made wherein the structural support is provided by vertically extending hollow tubes on the interior of the bundle of fuel rods. These hollow tubes serve as guides for control rod elements and are secured to upper and lower end plates or end fittings. The fuel rods are aligned and supported within the structural framework formed by the guide tubes and end plates by spacer grids which are attached to the guide tubes and provide both lateral and to some extent axial restraint to the fuel rod. The fuel assemblies themselves are held within the core by upper and lower core support structures normally comprised of upper and lower core support plates positioned transversely of the reactor vessel. The upper and lower end fittings of the fuel assemblies, and the upper and lower core support and alignment plates are provided with means for interfacing with one another in order to provide the necessary support and alignment of the fuel assemblies.
One previously developed means for supporting and aligning the fuel assemblies in the core was to provide the lower core support plate with upwardly facing pins which slidingly fit into extensions of the downwardly directed control rod guide tubes. Alternatively, a previously known arrangement was to provide holes within the lower core support plate adapted to receive downwardly directed pin-like extensions at the lower ends of the control rod guide tubes. The interface between the upper end fitting of the fuel assembly and the upper guide plate also conventionally comprise a plurality of holes within the upper guide plate adapted to receive upwardly extending extensions of the control rod guide tubes or of portions of the fuel assembly upper end fittings.
In addition to the aligning holes or pins formed in or carried by the upper and lower support plates, these plates also contain flow holes to permit passage of coolant through the plates so that coolant may enter the core at the bottom and exit the core at the top. One of the difficulties with this conventional design is that these upper and lower core alignment and support plates are difficult and expensive to manufacture. In addition, the provision of a plurality of upwardly or downwardly extending pins within the periphery of the cross-section of the fuel assembly creates local areas where coolant flow is impeded from flowing through the core support plates and through the fuel assemblies themselves. Accordingly, a core support and fuel assembly design is desired which avoids these difficulties and which is easier and less expensive to manufacture.